Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing sides of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof synonymously refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging). Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed.
Metal-based active materials can often be desirable for use in flow batteries and other electrochemical energy storage systems. Although non-ligated metal ions (e.g., dissolved salts of a redox-active metal) can be used as an active material, it can often be more desirable to utilize coordination complexes for this purpose. As used herein, the terms “coordination complex,” “coordination compound,” “metal-ligand complex,” or simply “complex” synonymously refer to a compound having at least one covalent bond formed between a metal center and a donor ligand. The metal center can cycle between an oxidized form and a reduced form in an electrolyte solution, where the oxidized and reduced forms of the metal center represent states of full charge or full discharge depending upon the particular half-cell in which the coordination complex is present. In certain instances, additional electrons can be transferred through the oxidation or reduction of one or more of the molecules constituting the ligands.
Titanium complexes can be particularly desirable active materials for use in flow batteries and other electrochemical energy storage systems, since such metal complexes can provide good half-cell potentials (e.g., less than −0.3 V) and current efficiencies exceeding 85% at high current density values (e.g., greater than 100 mA/cm2). Various titanium catecholate complexes can be especially desirable active materials in this regard, since they are relatively stable complexes and have a significant degree of solubility in aqueous media. Although various methods utilizing organic solvents are available for synthesizing titanium catecholate complexes (also referred to herein as titanium catechol complexes, titanium catecholate coordination compounds, catechol complexes of titanium, and/or similar terms), none are presently viable for producing the significant quantities of these complexes needed to support commercial-scale energy storage applications. In addition, residual organic solvents from currently employed syntheses of titanium catecholate complexes can become incorporated in aqueous electrolyte solutions in which the complexes are present, which can be undesirable in various instances. Certain residual organic solvents, for example, can cause membrane swelling in a flow battery, which can compromise the flow battery's operation. In addition, residual organic solvents can present environmental or safety concerns in some instances.
In addition, titanium catecholate complexes are usually synthesized in a salt form for incorporation in aqueous electrolyte solutions. In such salt forms, the titanium catecholate complex itself bears a formal negative charge and one or more positively charged counterions are present to maintain charge balance. If extraneous salts (i.e., salts not associated with the titanium catecholate complex) are also present in an aqueous electrolyte solution, the solubility of the complex can be undesirably lowered through a common ion effect. Since most conventional syntheses of titanium catecholate complexes liberate at least one byproduct species that can readily lead to extraneous salt formation, it can be difficult to realize maximized solubility levels for these complexes when conventional synthesis conditions are employed. The decreased solubility values can undesirably impact energy density values and other parameters of interest.
In view of the foregoing, improved methods for synthesizing titanium catecholate complexes to support their use as active materials in energy storage applications would be highly desirable in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.